1. Field of the Invention
The present invention relates to a storage control unit which, when a small sub-screen is displayed in an original screen (main screen) of a display unit used for televisions and computers, writes/reads sub-screen data to be displayed on such sub-screen.
2. Description of the Prior Art
FIG. 6 shows an example where a sub-screen is displayed in a main screen of a display unit such as a television. The main screen 600 is displayed synchronizing with the horizontal synchronizing signal MHS and the vertical synchronizing signal MVS for the main screen. A sub-screen 700 of a smaller scale is displayed in this main screen 600. Video signals for the sub-screen 700 are converted to digital signals and stored as digital data in a storage incorporated in the display unit (described later). When the sub-screen 700 is displayed in the main screen 600, the sub-screen reading start signal VRS for vertical direction and the sub-screen reading start signal HRS for horizontal direction are raised to HIGH level and the digital signals stored in the storage are read out. The digital signals read out are converted to analogue signals before output so as to cause the sub-screen to be displayed.
FIG. 7 shows an example of a storage control unit for sub-screen data provided for sub-screen display as described above. Referring to FIG. 7, the unit is provided with a reloadable storage 300 to store sub-screen video signals which have been converted to digital data. A row decoder 310 decodes the output from a writing row counter 330 or a reading row counter 340 sent via a row switching circuit 320 so as to generate a row address signal AV. A column decoder 350 decodes the output from a writing column counter 370 or a reading column counter 380 sent via a column switching circuit 360 so as to generate a column address signal AH.
Since the sub-screen to be displayed in the main screen must be reduced to a small size, a reduction processing is required at this storage control unit for sub-screen data when writing data to the storage 300: the writing row counter 330 counts the sub-screen writing signal SWSn obtained by dividing the sub-screen horizontal synchronizing signal SHS by the reduction ratio in vertical direction (1/n). The writing column counter 370 counts the writing clock WCKn obtained by dividing the reading clock RCK by the reduction ratio (1/n).
On the other hand, when data in the storage 300 is read for display of a sub-screen, the reading row counter 340 counts the horizontal synchronizing signal MHS for the main screen. The reading column counter 380 counts the reading clock RCK. The row switching circuit 310 and the column switching circuit 360 switch the address signals for digital video data for the sub-screen between reading from and writing to the storage 300 using the switching signal RWC.
FIG. 8 shows the specific configuration of a writing row counter 330, a reading row counter 340 and a row switching circuit 320 for a storage control unit described above. Both of the writing row counter 330 and the reading row counter 340 comprise a series of connected flip-flops 120a to 120n. A set of the writing column counter 370, the reading column counter 380 and the column switching circuit 360 has the same configuration.
Referring to FIG. 7, brief description is given below for the operation of the storage control unit for sub-screen data. When the switching signal RWC has a logical value "1", a writing row address signal and a writing column address signal are output. The former is the decoded output from the writing row counter 330, which has counted the sub-screen writing signal SWSn obtained by dividing the sub-screen horizontal synchronizing signal SHS by the reduction ratio (1/n). The latter is the decoded output from the writing column counter 370, which has counted the writing clock WCKn obtained by dividing the reading clock RCK by the reduction ratio (1/n). To the area in the storage 300 defined by these low address signal and the column address signal, the digital video data, i.e. the digitally converted video signals for the sub-screen are written.
On the other hand, when the switching signal RWC has a logical value "0", the reading row counter 340 counts the horizontal synchronizing signal MHS for the main screen upon input of the sub-screen reading start signal VRS for vertical direction- The reading column counter 380 counts the reading clock RCK upon input of the horizontal sub-screen reading start signal HRS. The counted values are decoded to provide row address and column address signals. From the area in the storage 300 defined by these address signals, digital video data are read and converted to analogue signals for display of a sub-screen in the main screen. The vertical position of the sub-screen on main screen is controlled by the sub-screen reading start signal VRS and its horizontal position by the sub-screen reading start signal HRS.
The conventional storage control unit for sub-screen data as described above has the outputs from the writing column counter and the writing row counter and those from the reading column counter and the reading row counter decoded as they are before input to the storage. This results in that the area in the storage cannot be divided, and the number of sub-screens to be displayed on main screen is limited to one.